DVB-TDVB-T is an abbreviation for "
Digital Video BroadcastingDigital Video Broadcasting —
Terrestrial"; it is the DVB European-based consortium standard for the
broadcast transmission of digital terrestrial television that was
first published in 1997[1] and first broadcast in the UK in 1998.[1]
This system transmits compressed digital audio, digital video and
other data in an MPEG transport stream, using coded orthogonal
frequency-division multiplexing (
COFDMCOFDM or OFDM) modulation. It is also
the format widely used worldwide (including North America) for
Electronic News Gathering for transmission of video and audio from a
mobile newsgathering vehicle to a central receive point.

Contents

1 Basics
2 Technical description of a
DVB-TDVB-T transmitter
3 Technical description of the receiver
4 Countries and territories using
DVB-TDVB-T or DVB-T2

4.1 Americas
4.2 Europe
4.3 Oceania
4.4 Asia
4.5 Africa

5 See also
6 Notes
7 References
8 External links

Basics[edit]
Rather than carrying one data carrier on a single radio frequency (RF)
channel,
COFDMCOFDM works by splitting the digital data stream into a large
number of slower digital streams, each of which digitally modulates a
set of closely spaced adjacent sub-carrier frequencies. In the case of
DVB-T, there are two choices for the number of carriers known as
2K-mode or 8K-mode. These are actually 1,705 or 6,817 sub-carriers
that are approximately 4 kHz or 1 kHz apart.
DVB-TDVB-T offers three different modulation schemes (QPSK, 16QAM, 64QAM).
DVB-TDVB-T has been adopted or proposed for digital television broadcasting
by many countries (see map), using mainly VHF 7 MHz and UHF
8 MHz channels whereas Taiwan, Colombia,
PanamaPanama and Trinidad and
Tobago use 6 MHz channels. Examples include the UK's Freeview.
The
DVB-TDVB-T Standard is published as EN 300 744, Framing structure,
channel coding and modulation for digital terrestrial television. This
is available from the
ETSIETSI website, as is
ETSIETSI TS 101 154,
Specification for the use of
VideoVideo and Audio Coding in Broadcasting
Applications based on the
MPEG-2MPEG-2 Transport Stream, which gives details
of the DVB use of source coding methods for
MPEG-2MPEG-2 and, more recently,
H.264/MPEG-4 AVCH.264/MPEG-4 AVC as well as audio encoding systems. Many countries
that have adopted
DVB-TDVB-T have published standards for their
implementation. These include the
D-book in the UK, the Italian
DGTVi,[2] the
ETSIETSI E-Book and the Nordic countries and Ireland NorDig.
DVB-TDVB-T has been further developed into newer standards such as DVB-H
(Handheld), which was a commercial failure and is no longer in
operation, and DVB-T2, which was initially finalised in August 2011.
DVB-TDVB-T as a digital transmission delivers data in a series of discrete
blocks at the symbol rate.
DVB-TDVB-T is a
COFDMCOFDM transmission technique
which includes the use of a Guard Interval. It allows the receiver to
cope with strong multipath situations. Within a geographical area,
DVB-TDVB-T also allows single-frequency network (SFN) operation, where two
or more transmitters carrying the same data operate on the same
frequency. In such cases the signals from each transmitter in the SFN
needs to be accurately time-aligned, which is done by sync information
in the stream and timing at each transmitter referenced to GPS.
The length of the Guard Interval can be chosen. It is a trade-off
between data rate and SFN capability. The longer the guard interval
the larger is the potential SFN area without creating intersymbol
interference (ISI). It is possible to operate SFNs which do not
fulfill the guard interval condition if the self-interference is
properly planned and monitored.
Technical description of a
DVB-TDVB-T transmitter[edit]

With reference to the figure, a short description of the signal
processing blocks follows.

Source codingSource coding and
MPEG-2MPEG-2 multiplexing (MUX)
Compressed video, compressed audio, and data streams are multiplexed
into MPEG program streams (MPEG-PS's). One or more MPEG-PS's are
joined together into an
MPEG transport streamMPEG transport stream (MPEG-TS); this is the
basic digital stream which is being transmitted and received by TV
sets or home Set Top Boxes (STB). Allowed bitrates for the transported
data depend on a number of coding and modulation parameters: it can
range from about 5 to about 32
Mbit/s (see the bottom figure for a
complete listing).
Splitter
Two different MPEG-TSs can be transmitted at the same time, using a
technique called Hierarchical Transmission. It may be used to
transmit, for example a standard definition
SDTVSDTV signal and a high
definition
HDTV signal on the same carrier. Generally, the
SDTVSDTV signal
is more robust than the
HDTV one. At the receiver, depending on the
quality of the received signal, the STB may be able to decode the HDTV
stream or, if signal strength lacks, it can switch to the
SDTVSDTV one (in
this way, all receivers that are in proximity of the transmission site
can lock the
HDTV signal, whereas all the other ones, even the
farthest, may still be able to receive and decode an
SDTVSDTV signal).
MUX adaptation and energy dispersal
The MPEG-TS is identified as a sequence of data packets, of fixed
length (188 bytes). With a technique called energy dispersal, the byte
sequence is decorrelated.
External encoder
A first level of error correction is applied to the transmitted data,
using a non-binary block code, a
Reed-Solomon RS (204, 188) code,
allowing the correction of up to a maximum of 8 wrong bytes for each
188-byte packet.
External interleaver
Convolutional interleaving is used to rearrange the transmitted data
sequence, in such a way that it becomes more rugged to long sequences
of errors.
Internal encoder
A second level of error correction is given by a punctured
convolutional code, which is often denoted in STBs menus as FEC
(Forward error correction). There are five valid coding rates: 1/2,
2/3, 3/4, 5/6, and 7/8.
Internal interleaver
Data sequence is rearranged again, aiming to reduce the influence of
burst errors. This time, a block interleaving technique is adopted,
with a pseudo-random assignment scheme (this is really done by two
separate interleaving processes, one operating on bits and another one
operating on groups of bits).
Mapper
The digital bit sequence is mapped into a base band modulated sequence
of complex symbols. There are three valid modulation schemes: QPSK,
16-QAM, 64-QAM.
Frame adaptation
the complex symbols are grouped in blocks of constant length (1512,
3024, or 6048 symbols per block). A frame is generated, 68 blocks
long, and a superframe is built by 4 frames.
Pilot and TPS signals
In order to simplify the reception of the signal being transmitted on
the terrestrial radio channel, additional signals are inserted in each
block. Pilot signals are used during the synchronization and
equalization phase, while TPS signals (Transmission Parameters
Signalling) send the parameters of the transmitted signal and to
unequivocally identify the transmission cell. The receiver must be
able to synchronize, equalize, and decode the signal to gain access to
the information held by the TPS pilots. Thus, the receiver must know
this information beforehand, and the TPS data is only used in special
cases, such as changes in the parameters, resynchronizations, etc.

Spectrum of a
DVB-TDVB-T signal in 8k mode (note the flat-top
characteristics)

OFDM Modulation
The sequence of blocks is modulated according to the OFDM technique,
using 1705 or 6817 carriers (2k or 8k mode, respectively). Increasing
the number of carriers does not modify the payload bit rate, which
remains constant.
Guard interval insertion
to decrease receiver complexity, every OFDM block is extended, copying
in front of it its own end (cyclic prefix). The width of such guard
interval can be 1/32, 1/16, 1/8, or 1/4 that of the original block
length.
Cyclic prefix is required to operate single frequency
networks, where there may exist an ineliminable interference coming
from several sites transmitting the same program on the same carrier
frequency.
DAC and front-end
The digital signal is transformed into an analogue signal, with a
digital-to-analog converter (DAC), and then modulated to radio
frequency (VHF, UHF) by the RF front end. The occupied bandwidth is
designed to accommodate each single
DVB-TDVB-T signal into 5, 6, 7, or 8
MHz wide channels. The base band sample rate provided at the DAC input
depends on the channel bandwidth: it is

f

s

=

8
7

B

displaystyle f_ s = frac 8 7 B

samples/s, where

B

displaystyle B

is the channel bandwidth expressed in Hz.

Available bit rates (Mbit/s) for a
DVB-TDVB-T system in 8 MHz channels

Modulation
Coding rate
Guard interval

1/4
1/8
1/16
1/32

QPSK
1/2
4.976
5.529
5.855
6.032

2/3
6.635
7.373
7.806
8.043

3/4
7.465
8.294
8.782
9.048

5/6
8.294
9.216
9.758
10.053

7/8
8.709
9.676
10.246
10.556

16-QAM
1/2
9.953
11.059
11.709
12.064

2/3
13.271
14.745
15.612
16.086

3/4
14.929
16.588
17.564
18.096

5/6
16.588
18.431
19.516
20.107

7/8
17.418
19.353
20.491
21.112

64-QAM
1/2
14.929
16.588
17.564
18.096

2/3
19.906
22.118
23.419
24.128

3/4
22.394
24.882
26.346
27.144

5/6
24.882
27.647
29.273
30.160

7/8
26.126
29.029
30.737
31.668

Technical description of the receiver[edit]
The receiving STB adopts techniques which are dual to those ones used
in the transmission.

Front-end and ADC: the analogue RF signal is converted to base-band
and transformed into a digital signal, using an analogue-to-digital
converter (ADC).
Time and frequency synchronization: the digital base band signal is
searched to identify the beginning of frames and blocks. Any problems
with the frequency of the components of the signal are corrected, too.
The property that the guard interval at the end of the symbol is
placed also at the beginning is exploited to find the beginning of a
new OFDM symbol. On the other hand, continual pilots (whose value and
position is determined in the standard and thus known by the receiver)
determine the frequency offset suffered by the signal. This frequency
offset might have been caused by Doppler effect, inaccuracies in
either the transmitter or receiver clock, and so on. Generally,
synchronization is done in two steps, either before or after the FFT,
in such way to resolve both coarse and fine frequency/timing errors.
Pre-FFT steps involve the use of sliding correlation on the received
time signal, whereas Post-FFT steps use correlation between the
frequency signal and the pilot carriers sequence.
Guard interval disposal: the cyclic prefix is removed.
OFDM demodulation: this is achieved with an FFT.
Frequency equalization: the pilot signals are used to estimate the
Channel Transfer Function (CTF) every three subcarriers. The CTF is
derived in the remaining subcarriers via interpolation. The CTF is
then used to equalize the received data in each subcarrier, generally
using a Zero-Forcing method (multiplication by CTF inverse). The CTF
is also used to weigh the reliability of the demapped data when they
are provided to the Viterbi decoder.
Demapping: since there are Gray-encoded
QAMQAM constellations, demapping
is done in a "soft" way using nonlinear laws that demap each bit in
the received symbol to a more or less reliable fuzzy value between -1
and +1.
Internal deinterleaving
Internal decoding: uses the Viterbi algorithm, with a traceback length
larger than that generally used for the basic 1/2 rate code, due to
the presence of punctured ("erased") bits.
External deinterleaving
External decoding
MUX adaptation
MPEG-2MPEG-2 demultiplexing and source decoding